In an electrical system with a DC link, an electrical power source produces electrical power to a DC link to which one or more electrical motors are connected for driving a load. In such an electrical system, the electrical power source is, for example, a generator that is driven with a main power source, for example, a diesel engine. The diesel engine rotates the generator, and the generator output is rectified and fed to the DC link.
The electrical motors connected to the DC link may be connected to the DC link by using a DC to AC converter, for example, an inverter. Such an inverter takes power from the DC link and controls the motor in a desired manner. In electrical systems with DC links, the motors connected to the DC link may be in such a use in which the motors need to be accelerated and decelerated more or less periodically. The deceleration of the motors can be carried out using regenerative braking. In regenerative braking, the inverter is controlled in such a way that braking power is fed back to the DC link. The ability to feed power back to the DC link by using kinetic energy of the load enhances the energy efficiency because less power is required from the main power source.
An electrical system using a DC link can have multiple inverters driving motors. Further, the power rating of the largest motors can be considerable compared with the capacity of the main power source, and the temporary power consumption of the loads connected to the DC link can be higher than the capacity of the main power source. For that reason and to further improve the efficiency of the system, the DC link may be equipped with electricity storage means, for example, batteries, capacitors or super capacitors. Such electricity storage means are connected to the DC link by using DC to DC converters that can transfer power in both directions. The energy storages can be used for storing energy that can be then used for powering the loads connected to the DC link. Thus, the regenerative power can be used at the same time by other equipment or stored to the electricity storage means.
In the case where the regenerative power fed to the DC link is higher than the consumption of the other loads and energy storages are full or power feed to the energy storage cannot be increased, the regenerative power fed to the DC link has to be dissipated. The dissipation of power can be carried out with brake choppers that are connected to the DC link and that feed power to a resistive component which dissipates excessive energy to heat.
The above-described electrical systems with DC links are used or can be used in connection with different diesel-electric or hybrid-electric vehicles or working machines, for example, off-road mining trucks, dumpers, wheel loaders, excavators and drilling rigs. Further applications employing such electrical systems include container handling machinery, different marine applications and on-road trucks and busses.
The control of individual apparatuses connected to a common DC link is carried out by using a higher-level controller structure, for example, a PLC (programmable logic controller). The controller is connected to each of the devices in the system by using a communications protocol, for example, profibus or CAN. The higher-level controller controls each converter of the system separately by using measured DC voltage of the DC link and the power balance of the system. The problem relating to known system relates to voltage control of the DC link voltage. As the higher-level controller is connected to the system by using communications interface, the control delays can lead to tripping of the whole system. If, for example, the power of one load is suddenly changed, the power balance in the system is disturbed. Due to communication delays in the power balance controller of the higher-level controller, the controller does not have enough time to react to the abruptly changed conditions and to adjust the reference values accordingly for the other converters of the system, and the whole electrical system can collapse.